The capability of computers to communicate with one another has become a basic attribute of modern information processing. There is an ongoing proliferation of user applications that depend upon the ability of a computer running one of the user applications to send and receive data to and from other computers. The communication capability is necessary for the user application to be able to complete the task for which the application was developed or to communicate information to other users within a group or organization. A particular application may be designed, for example, to call subroutines running on another computer for certain data processing functions or to access a remote database to obtain input data or to store results.
An important objective in providing a communication capability among computers is to make all of the database and processing resources of the group or organization available to each user in the group or organization. In response to the growing need for sophisticated communication capabilities among computers, network, routing and bridging protocols such as IP, DECnet, OSI, standard ISO 8473, etc. have been developed to control data transmissions between computers linked to one another in a network. The various protocols are implemented in transmission services used to couple computers to one another.
Each protocol is typically defined in terms of a number of layers, with each layer relating to a certain aspect of the functionality required for data transmissions throughout a network. For example, the first three layers are defined as a physical layer, a data link layer and a network layer. The physical layer is directed to the physical and electrical specifications of a physical link, for example, a bus, that couples the computers of a network to one another. The physical layer controls bit transmissions through the link so that a series of bits of a data packet can be communicated from one computer on the network to another computer on the network. The physical layer will set the voltage levels for logical ones and zeros, the timing of stable bit information on a physical link and so on, as necessary to transmit the bits of a data packet over the physical link.
The data link layer is directed to the packaging or framing of bits received in a data transmission into a defined packet that is free of transmission errors. The data link layer creates and recognizes boundaries between bits to define bit fields. The bits between boundaries provide structure and meaning to the bits of a data packet. For example the data packet can include a header, comprising the first n bits of a transmission, for computer source and destination information, the length of the data packet in bytes, the network protocol being used and so on. The header can be followed by framed bytes of data comprising the actual message being communicated between two or more computers on the network.
The network layer is directed to the control of routing information required to direct a message from a source computer to a destination computer of the network. Each protocol will define the length and content for a network address to uniquely identify each source or destination of data packets and the processing scheme for routing a data packet through a network. The network address information is in one of the bit fields of a data packet, as framed by the data link layer processing scheme defined by the protocol.
Networks are generally arranged into local area networks (LANs) and wide area networks (WANs). ALAN couples computers that are physically located relatively close to one another, as for example in the same building. A WAN couples computers that may be located in different cities or indeed on different continents. A WAN usually includes components such as a router to receive a data packet from a computer on one LAN and to forward the data packet to a computer on another LAN.
The router processes a network address of a data packet according to a protocol implemented in the transmission service used on the network to determine how the data packet is to be routed, e.g., to another router directly coupled to the receiving LAN or to an intermediate router, etc. Each router stores information on the topology of the network for use in routing each data packet.
A bridge is another component that may be utilized to transfer data packets from computers of one LAN to computers of another LAN. A bridge intercouples different LANs and monitors communications within and between the LANs to "learn" source and destination address information. The bridge continuously learns and "ages" address information to accommodate the transfer of a data packet between the LANs.
Typically, each of the bridge and router components have access to a look-up database that contains bridging or routing information, respectively, relating to the network addresses of the sources and destinations of data in the particular network being serviced by the respective router or bridge. An address recognition engine can be implemented as a resource for a router or bridge for access to the look-up database.
The router or bridge operates to extract the network address from the header of a data transmission received at the router or bridge and inputs the address to the address recognition engine. The address recognition engine utilizes the input network address as an index for traversal of the look-up database to locate an entry corresponding to the network address. The entry contains protocol information required by the router or bridge for directing the data transmission to the designated destination.
As should be understood, the nature and content of the information that would be stored in a database entry depends upon whether a router or bridge were implemented in the network due to the differing operating characteristics of routing and bridging operations. Moreover, the size and format of the network address and the nature and content of the network information related to the network address in a particular network are defined by the network protocol implemented in the network. Accordingly, the structure of a look-up database must be designed to accommodate either bridging or routing as well as the address length and information content requirements of the protocol being utilized in the network were the bridge or router is operating. Thus, a look-up database designed to support a router in a network implementing, e.g., a DECnet protocol would not be suitable for use by a bridge, and so on.